<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <style>
        .fa{
            padding: 10px;
            background-color: #cccccc;
            overflow: hidden;
        }
        .ch1{
            height: 100px;
            background-color: red;
            width: 50%;
            float: left;
        }
        .ch2{
            height: 100px;
            background-color: blue;
            width: 50%;
            float: left;

            border: 1px solid purple;

            box-sizing: border-box;
        }
    </style>
    <div class="fa">
        <div class="ch1"></div>
        <div class="ch2"></div>
    </div>
</body>
</html>